1. Field of the Invention
The present invention relates to a process for cleaning a harmful gas. More particularly, it pertains to a process for cleaning a nitrogen fluoride such as nitrogen trifluoride which is used or generated in the semiconductor manufacturing industry.
With the continuous development of the semiconductor industry, there has been a steady rise in recent years in the amount of nitrogen trifluoride which is used in the dry etching of silicon and silicon oxide, as a gas for cleaning the chamber of a CVD apparatus and the like. Nitrogen trifluoride gas is sparingly soluble in water and rather stable at room temperature with little reactivity with an acid or an alkali, but it is highly toxic and exerts an adverse influence on human bodies and the environment since the maximum permissible concentration thereof in the atmosphere is reported as being 10 ppm. It is therefore, necessary to clean a nitrogen trifluoride-containing gas after being used in a semiconductor production process prior to the discharge in the atmosphere.
In spite of its being stable at ordinary temperature, nitrogen trifluoride generates dinitrogen tetrafluoride, dinitrogen difluoride, dinitrogen hexafluoride, fluorine and the like due to heat, discharge, etc. in etching and cleaning processes, and each of them must be removed as well because of its toxicity is stronger than that of nitrogen trifluoride.
2. Description of the Related Art
As the method of removing a nitrogen fluoride contained in a mixed gas, there have heretofore been proposed (1) a process wherein the gas is brought into contact with metallic silicon at 100.degree. C. or higher (Japanese Patent Application Laid-Open No. 12322/1988), (2) a process wherein the gas is brought into contact with metallic titanium at 200.degree. C. or higher (Japanese Patent Publication No. 48571/1988), (3) a process wherein the gas is brought into contact with Si, B, W, Mo, V, Se, Te, Ge or a non-oxide based compound of any of them at 200.degree. to 800.degree. C. (Japanese Patent Publication No. 48570/1988), (4) a process wherein the gas is brought into contact with a metal halogenide capable of transhalogenation with nitrogen fluoride (Japanese Patent Publication No. 48569/1988), (5) a process wherein the gas is brought into contact with the oxide of a transition metal such as Fe, Mn or Cu at 250.degree. C. or higher (Japanese Patent Application Laid-Open No. 181316/1991), (6 ) a process wherein the gas is brought into contact with activated carbon at 300.degree. to 600.degree. C. (Japanese Patent Application Laid-Open No. 237929/1987), (7) a process wherein the gas is brought into contact with a composition comprising as principal components Ni, Fe, Co or a noble metal such as Pt, Rh and Pd and at least one from alumina and silica at 200.degree. C. or higher (Japanese Patent Application Laid-Open No. 27303/1987).
Nevertheless, any of the above-proposed processes is insufficient in gas cleaning capability and besides suffers the disadvantage as described hereunder. A volatile fluoride is produced in the processes (1), (2) & (3), a halogen such as chlorine is formed in, the process (4) and a nitrogen oxide is generated in the process (5), each incurring an expense in the treatment of itself. It is necessary in the processes (2) & (3) to heat the reaction system to 300.degree. C. or higher in order that the produced fluoride may not cover the surface of the reaction agent and thus hinder the cleaning reaction. The process (6) involves the danger of explosion due to a violent reaction at an elevated temperature and the problem of byproducing carbon tetrafluoride that is relatively stable and difficult to remove. In the process (7), a harmful gas is not byproduced but a high temperature is required for attaining sufficient cleaning capability and in the case of nickel (Ni), for example, for the purpose of achieving practical performance, the reaction system needs to be heated to 400.degree. C. or higher, that is, much higher than 200 .degree. C. at which the decomposition activity is insufficient and besides, the surface of the reaction agent is covered with the fluoride with the progress of the reaction, thereby failing to assure sufficient cleaning capability.